Read/write optical pickups have been used in optical readers for read-only CDs, LDs, DVDs and other similar optical discs and also in optical reader/writer for write once or rewriteable optical discs, such as CD-Rs, CD-RWs, DVD-Rs, DVD-RWs, DVD-RAMs, and mini discs (MDs).
Optical discs as optical storage media have a transparent substrate with a predetermined thickness to cover their recording surface for protective purposes. An optical pickup as information readout means reads the optical disc by the intensity of reflection when a read beam is shone on the recording surface through the transparent substrate.
However, it is difficult in manufacturing to fabricate the transparent substrates in all optical discs with the same specified thickness; irregularities of a few or 10 to 40 micrometers are typical. The irregular thickness of the transparent substrate results in spherical aberration which in turn can markedly reduce the amplitude of an information readout signal and/or tracking error signal. This entails inaccurate information readout. Specifically, when changing optical discs, there will likely be a difference in thickness between the transparent substrates. The difference translates into a change in spherical aberration and without taking any measures, causes a fall in information readout accuracy.
The problem is addressed in Japanese published patent application 2001-222838 (Tokukai 2001-222838; published on Aug. 17, 2001). Tokukai 2001-222838 discloses a method involving spherical aberration correction means which corrects spherical aberration in an optical system in accordance with the magnitude of the spherical aberration, whereby the spherical aberration is corrected by observing the amplitude of a tracking error signal while varying the magnitude of correction so that the spherical aberration correction means is supplied with an final spherical correction assisting quantity which is the magnitude of correction at which the amplitude assumes a maximum value.
The spherical aberration correction means, according to Tokukai 2001-222838, is a liquid crystal panel in which a circular band of transparent electrode is formed on a liquid crystal layer filled with birefringent liquid crystal. The magnitude of correction is varied in accordance with the voltage applied to the transparent electrode. The liquid crystal panel as the spherical aberration correction means is located on the optical axis of a laser producing element. The laser beam radiating from the laser producing element thus develops a phase difference at its wavefront, passes through it, and converges on the recording surface of an optical disc.
Japanese published patent application 2000-11388 (Tokukai 2000-11388; published on Jan. 14, 2000) discloses a method of correcting spherical aberration in an optical system in accordance with the magnitude of the spherical aberration, using prerecorded prepit data on an optical disc as a reference signal. The spherical aberration is corrected by observing the amplitude of the reference signal while varying the magnitude of correction so as to produce an final spherical correction assisting quantity which will be the magnitude of correction at which the amplitude assumes a maximum value.
Tokukai 2000-11388, a conventional art, has a problem that it is not applicable to discs without prepit signals. In addition, the prepit signal is stored, for example, in sector marks which in general give such a small amount of data that the areas may not be sufficient to enable accurate observation of the magnitude of the correction of the spherical aberration. Moreover, in cases of write once, rewriteable, and other writeable optical discs, if the data derived from the prepit signal is applied to correction for storage areas, accurate correction is likely to be impossible because of, strictly speaking, different storage mechanisms: In prepit areas, recording utilizes the intensity of reflection which decreases when light diffracts in pit sections. In storage areas, recording utilizes the presence/absence of an increase in absorption by storage sections (tint signal).
Japanese published patent application 64-27030/1989 (Tokukaisho 64-27030; published on Jan. 30, 1989) provides exemplary write power and focus offset correction for writeable optical discs. According to Tokukaisho 64-27030, focus offset of an optical disc is corrected using an information readout signal as a reference signal. Write power is varied from one sector to another. After writing, the sectors are simultaneously read to find the sector with an optimal result. The write power for that sector is the optimal write power.
A disadvantage of the Tokukai 2001-222838 method above is an extended time the method needs to determine the magnitude of correction, because the spherical aberration correction means needs to examine the magnitude of correction throughout the available range to find a magnitude of correction at which the amplitude of the tracking error signal is at a maximum. Further, at lower spherical aberrations, the amplitude of the tracking error signal changes less, which makes it impossible to accurately find the maximum amplitude in the presence of noise, external disturbance, and other factors.
Tokukaisho 64-27030, a conventional art, has a problem of an extended time it takes to write multiple sectors using multiple write powers and read all the sectors to find an optimal magnitude of correction.
An alternative to the prepit signal as the reference signal is a track cross signal obtained when the optical pickup crosses a track. With this technique, however, the signal level can show a maximum value when the spherical aberration and the focus offset are not completely eliminated. If this happens, conversion to an optimal condition cannot be achieved.
This particular problem will be described in more detail in reference to FIG. 18 which is a graph showing measurements of the levels of a reference signal in relation to two kinds of parameters, i.e. the spherical aberration and the focus offset. The measurements were performed by the inventors of the instant invention. The optical disc used contained a 0.1 mm thick transparent, polycarbonate substrate and has a track pitch of 0.32 μm and a disc groove depth of 21 nm. The pickup used in the measurement had a laser wavelength of 405 nm. The objective lens had a NA of 0.85.
FIG. 18 is a 2-dimensional map of a total of 6×11=66 data points, showing maximum amplitude values of a track cross signal. Six spherical aberrations from −80 mλ to +80 mλ were plotted on the horizontal axis, and 11 focus offsets from −0.22 to +0.22 μm were plotted on the vertical axis. “mλ” is a general unit for aberration. “λ” is the wavelength of a laser where 1 mλ=0.001λ. For example, λ=405 nm for a typical blue laser.
As obviously seen from FIG. 18, the reference signal is at a maximum in the left-falling region immediately surrounding the original point where either the spherical aberration or the focus offset is not 0. This shows that the reference signal can be at a maximum even when the lens-to-lens distance is not optimal and the lenses are out of focus. The track cross signal therefore cannot be used for accurate measurement of spherical aberration and focus offset.
The present invention has an objective to provide a spherical aberration correction method for an optical pickup capable of quick correction of spherical aberration using an accurate optimal magnitude of aberration correction without being affected by noise, external disturbance, and other factors, and also to provide such an optical pickup.
The present invention has another objective to provide a spherical aberration focus offset correction method for an optical pickup capable of quick and accurate correction of spherical aberration and focus offset for a writeable optical disc, and also to provide an optical pickup with such correction functions.